This disclosure of Japanese Patent Application No. 2001-254507 filed on Aug. 24, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to an in-cylinder injection type spark-ignition internal combustion engine.
2. Description of Related Art
An in-cylinder injection type spark-ignition internal combustion engine, in which a fuel is injected directly into each cylinder of the engine during a latter half of a compression stroke is known. The engine provides a fine combustible air-fuel mixture concentrated around a spark plug, which allows a stratified charge combustion that assures combustion of the air-fuel mixture within the cylinder while keeping an overall air-fuel ratio of the air-fuel mixture lean.
In such an in-cylinder injection type spark-ignition internal combustion engine, the spark plug is arranged at an upper portion nearly at the center of the cylinder, and a fuel injection valve is arranged at an upper portion of the cylinder. The fuel injected during the latter half of the compression stroke enters into a cavity formed in a part of a top face of a piston and is guided toward the spark plug along a configuration of the cavity. As a result, the combustible air-fuel mixture is concentrated around the spark plug. For that reason, the cavity is located closer to the fuel injection valve such that the center of the cavity deviates from the center of the piston.
In the case where the cavity is formed in the top face of the piston as described above, when the piston approaches the upper wall of the cylinder at the latter half of the compression stroke, an air stream is generated around the spark plug, flowing toward the cavity formed in the part of the top face of the piston from the other part of the top face of the piston where no cavity is formed.
The stratified charge combustion is required even at high engine speeds so as to improve the fuel efficiency. However, the air stream generated around the spark plug during the latter half of the compression stroke is further intensified at high engine speeds. Accordingly the air stream, thus, disperses the combustible air-fuel mixture formed around the spark plug. Thus, it is unlikely that the stratified charge combustion at high engine speeds is successfully accomplished
In Japanese Patent Laid-Open Publication No. 2000-97031, the ratio of volumes of the cylinder space above the cavity to the total volume of the cylinder is within a range between 0.4 and 0.6 at a top dead center (TDC) of the compression stroke. Even with the aforementioned arrangement, the intensified air stream is still generated around the spark plug at high engine speeds, failing to realize good stratified charge combustion.
The invention thus provides a successful stratified charge combustion at high engine speeds in an in-cylinder injection type spark-ignition internal combustion engine with the aid of a cavity formed in a top face of the piston.
An in-cylinder injection type spark-ignition internal combustion engine includes a spark plug disposed at an upper portion of a cylinder, a fuel injection valve through which a fuel is directly injected into the cylinder, and a piston having a top face and a cavity formed in a first part of the top face. The cavity is located closer to the fuel injection valve with respect to a center of the piston so as to bring the fuel injected through the fuel injection valve toward the spark plug. A space formed in the cylinder at a top dead center of a compression stroke of the piston includes a first space in which a center of the cavity is located, and a second space. The first space is located on one side of a plane that passes through a center axis of the spark plug in parallel with a center axis of the cylinder and is perpendicular to a radial direction in which the center of the cavity deviates from the center of the piston. The second space is located on the other side of the plane. The top face of the piston is designed so as to reduce a volumetric difference between the first space and the second space, thereby preventing generation of a strong air stream flowing between the first space and the second space at around the top dead center.
The volumetric ratio of the first space to the second space is set to be within a range between 6:4 and 5:5.
A protruding portion is formed on the top face of the piston along a peripheral portion of the cavity.